Subscriber lines of public or private communication networks provided for voice communication are increasingly being used also for data communication. Known examples of these are the transmission technologies summarized under the code xDSL (x Digital Subscriber Line), that permit very high data transmission rates. Widespread xDSL technologies are, for example, ADSL (Asynchronous Digital Subscriber Line), HDSL (High Bit Rate Digital Subscriber Line), VHDSL (Very High Bit Rate Digital Subscriber Line), VADSL (Very High Bit Rate Asymmetrical Digital Subscriber Line) or SDSL (Symmetric Digital Subscriber Line).
With the ADSL-Over-ISDN technologies, as they are known, that are widespread in public communication networks, a frequency range of 162-1102 kHz that can still be physically transmitted via the subscriber line is used for data transmission, in addition to a 0-120 kHz frequency range for an ISDN basic access reserved for voice communication. Because the given frequency ranges are disjunctive, voice and data can be transmitted independently and parallel to each other via a common subscriber line. For this purpose, voice terminals and data terminals are coupled via a frequency-separating filter, often known as a splitter, to the common subscriber line. A particular voice terminal is in this case connected via a S0 or a UK0 interface to the splitter.
Because, however, the ADSL-Over-ISDN technology is essentially designed for public communication networks and their subscriber lines and voice terminals, the set of performance features for voice terminals is substantially limited.